High resolution imaging of the body serves multiple purposes, including any of i) assessing tissue structures and anatomy; ii) planning and/or guiding interventions on localized regions of the body; and iii) assessing the result of interventions that alter the structure, composition or other properties of the localized region. High resolution imaging in this particular case refers to high frequency ultrasound and optical imaging methods. For the purposes of this invention, high frequency ultrasound typically refers to imaging with frequencies of greater than 3 MHz, and more typically in the range of 9 to 100 MHz. High frequency ultrasound is very useful for intravascular and intracardiac procedures. For these applications, the ultrasound transducers are incorporated into a catheter or other device that can be inserted into the body. By way of example, two particularly important implementations of high frequency ultrasound are intravascular ultrasound (IVUS), for imaging blood vessels, and intracardiac echocardiography (ICE) for imaging cardiac chambers. Both ICE and IVUS are minimally invasive, and involve placing one or more ultrasound transducers inside a blood vessel or cardiac chamber to take high quality images of these structures.
Optical imaging methods based on fiber optic technology used in the field of medicine include optical coherence tomography, angioscopy, near infrared spectroscopy, Raman spectroscopy and fluorescence spectroscopy. These modalities typically require the use of one or more optical fibers to transmit light energy along a shaft between an imaging site and an imaging detector. Optical coherence tomography is an optical analog of ultrasound, and provides imaging resolutions on the order of 1-30 microns, but does not penetrate as deeply into tissue as ultrasound in most cases. Fiber optics can also be used to deliver energy for therapeutic maneuvers such as laser ablation of tissue and photodynamic therapy. Additional forms of imaging related to this invention include angioscopy, endoscopy and other similar imaging mechanisms that involve imaging a site inside the patient using a probe to take pictures based on either the backreflection of light in the visible or infrared ranges of the spectrum. Further additional forms of high resolution imaging can use acoustic energy to create optical energy (sonoluminescence imaging) or optical energy to create acoustic energy (photoacoustic imaging).
High resolution imaging means have been implemented in many forms for assessing several different regions of mammalian anatomy, including the gastrointestinal system, the cardiovascular system (including coronary, peripheral and neurological vasculature), skin, eyes (including the retina), the genitourinary systems, breast tissue, liver tissue and many others. By way of example, imaging of the cardiovascular system with high frequency ultrasound or optical coherence tomography has been developed for assessing the structure and composition of arterial plaque. High resolution imaging has been used to measure vessel or plaque geometry, blood flow through diseased arteries and the effect of interventions on arterial plaque (such as by atherectomy, angioplasty and/or stenting). Attempts have also been made using high resolution imaging to identify vascular lesions that have not led to clinical symptoms, but are at increased risk of rupturing or eroding and causing an acute myocardial infarction. These so-called “vulnerable plaques” are an area of intense interest as the prospect of treating such plaques to pre-empt adverse clinical events is conceptually appealing. However, no particular imaging modality has as of yet demonstrated efficacy in this regard.
Chronic total occlusions are a specific subset of vascular lesions where the entire lumen of the vessel has been occluded (based on the angiographic appearance of the lesion) for over approximately one month. Most intravascular imaging modalites are “side-viewing” and require passage of an intravascular imaging device through a lesion. In order to image chronic total occlusions, methods of high resolution imaging would be more useful if they were adapted to a “forward-looking” rather than “side-viewing” configuration.
Another area of increasing interest is the use of image guidance for procedures in structural heart disease and electrophysiology procedures. It is often necessary to place catheters within specific positions in the cardiac chambers in order to perform a therapeutic maneuver, such as the implantation of a device (such as a closure device for patent foramen ovales, valvular repair or replacement devices, left atrial appendage closure devices) or the placement of a therapeutic catheter (such as an ablation or cryotherapy catheter). It may also be necessary to guide intermediate steps in a procedure, such as crossing the atrial septum of the heart. The use of high resolution imaging can facilitate these steps. Intracardiac echo (ICE), currently performed using linear phased arrays, is one such technology currently used for this purpose.